I ask how far would you normally be casting the line? That will determine the average line mass outside of the rod; and more than anything else, this will determine the best weight fly line to get.

For example if you are going to normally cast the fly 40 feet, that would be about 8 foot of fly rod, 8 feet of leader and 24 or so feet of fly line. It is still not the theoretical 30 feet of line to "balance" the rod. My suspicion is that you will make 90+% of your casts will not require 30 feet of fly line. So get a 4 weight fly line. Unless you are going to routinely cast long, I'd get a DT 4 wt line.

I sometimes fish small flies down stream, or soft hackles, on my 4 weight, using a slack line mending technique, with a lot of line out, on smooth, glassy watered spring creeks. A weight forward line does not mend well slack line, not nearly as nice as a clean, freshly dressed Double Taper line. Nor does it float as well.

Go with a reel that will allow you to load 50 yds of backing and a DT 4 weight line.